I’d start by resetting the network settings on your iOS devices.
Changing channels can be tedious, especially with Sky routers and their tendency to crash. I’d start with 6 and see what happens.
My father has had an extender in the living room for 9 years. Absolutely convinced it was of benefit. I unplugged it; turned his router off auto and onto Ch. 11 and he has now conceded that all is working fine; Rick solid and arguably a tad faster.